For other host's information:
I recently had to cancel a reservation as a host due to illness with 2 days notice prior to the guest check-in.
I called Airbnb support and the call centre person (Justine) was very helpful and understanding.
She reviewed my host history and because I am a superhost, have never previously cancelled and generally have 5-star reviews, she waived all host penalties for cancellation and did not require supporting documentation (this was a 'one off').
She advised that if in future I needed to cancel, then I would need to provide a medical certificate from a personal doctor indicating that I would be unable host for the relevant period.
Hope this helps for other hosts in a similar situation.